BRPI0620785B1 - method for the recovery of rare metals in a zinc leaching process - Google Patents

Abstract

Method for the recovery of rare metals in a zinc leaching process The invention relates to a rare metal recovery process in connection with a zinc sulphite concentrate leaching process. Zinc sulfite concentrates usually also contain small amounts of rare metals such as indium and gallium. If the content of these metals in the raw material is high enough, their recovery may be economically interesting. In the method according to the invention, recovery of indium and other desirable rare metals occurs in a zinc leaching process in which at least part of the sulphite concentrate is directly leached without burning.

Description

METHOD FOR RECOVERY OF RARE METALS IN A PROCESS OF

ZINC Leaching

FIELD OF INVENTION

Zinc sulphite concentrates usually also contain small amounts of rare metals such as indium and gallium. If the content of these metals in the raw material is high enough, their recovery may be economically interesting. In the method according to the invention, recovery of indium and other desirable rare metals occurs in a zinc leaching process, in which at least part of the sulfitic concentrate is directly leached without burning.

BACKGROUND OF THE INVENTION The conventional method for treating a zinc sulfite concentrate is the burning of the concentrate, in which the sulfite concentrate is burned to zinc oxide, and the iron in the concentrate forms mainly zinc ferrite. Zinc oxide dissolves reasonably easily, such that in the first stage calcine undergoes leaching, which is called neutral leaching. Zinc ferrite remains undissolved in neutral bleach and in order to recover this zinc from ferrite a strong acid leach is often used. The zinc ferrite residue also contains the ferric iron residue precipitated in neutral leaching. The ferric iron residue itself contains, in addition to ferric hydroxide, aluminum hydroxide and co-precipitated rare metals such as gallium and indium. The ferrite residue may also be fed to a Waelz furnace, in which the zinc is evaporated, and is then oxidized to zinc oxide and fed back to the leaching process. Waelz oxide can also be treated in a separate process step for the purpose of recovering the other co-precipitated metals such as indium.

Nowadays the trend is increasingly towards processes in which at least part of the zinc sulfite concentrate is fed into the leach without burning. This enables the treatment of impure and finely granulated concentrates. A direct leaching process for zinc sulfide concentrates can be performed at both atmospheric pressure and leaching process pressure. However, zinc sulfide leaching requires a much higher acid concentration than that used for neutral calcine leaching, however, since elemental zinc manufacture almost always occurs electrolytically, the acid spent on electrolysis can be used for concentrate leaching. Zinc ferrite leaching requires the highest zinc concentration of all. Sulfide concentrate leaching can be combined with a process in which the leaching of the formed ferrite blends occurs as a strong acid leach and thus the leaching of ferrite is performed in connection with the concentrate leaching. In this case, a countercurrent leaching process is used, where in addition to a strong acid leaching stage that enables zinc ferrite leaching, there is also a weak acid leaching stage. A significant part of concentrate leaching occurs in its own right at the weak acid leaching stage. These types of methods are described, for example, in US Patent Nos. 6,475,450, 5,858,315 and 6,340,450 and in WO 2004/076698. The zinc concentrate may contain rare metals such as indium and gallium which are desirable to recover. One possible method for implementing the recovery of these metals is the processing of neutral leaching leachates in a Waelz oxide Waelz furnace and the leaching of this oxide, whereby the metals remaining in the oxide are returned to the solution and are subsequently returned to the solution. recovered in a liquid-liquid extraction. This type of indium and gallium recovery in connection with a Waelz oxide leaching process is known in the state of the art. This process is aided by the fact that these metals have already enriched Waelz oxide as they are co-precipitated with ferric hydroxide in neutral leaching. According to the method, zinc oxide containing a valuable metal is leached by sulfuric acid, whereby metals and zinc dissolve and lead and silver plus other inert compounds in the oxide remain in the residue. The solution is directed to indium extraction, where the indium is separated from zinc, and the zinc sulfate solution is directed to the neutral leaching step. If the concentrate contains gallium, its recovery occurs in principle during the recovery of indium, whereby indium and gallium are separated into their own phases.

PURPOSE OF THE INVENTION

When the unburned concentrate leaching is at least partially linked to the recovery of zinc from sulfide concentrate, there is currently no method by which indium and other rare metals are recovered from the concentrate than in direct leaching and resulting solution. The major problem with the recovery of these metals is that the solution that comes out of direct leaching containing rare metals is, in practice, inadequate with respect to the composition for conventional recovery processes of these metals. The method according to the invention enables the recovery of at least one of the rare metals contained in the concentrate, such as indium and gallium, in connection with direct leaching of zinc sulfide concentrate.

SUMMARY OF THE INVENTION

The main features of the invention will become apparent from the appended claims. The invention relates to a method for the recovery of at least one rare metal, such as indium and / or gallium, in connection with zinc sulfide concentrate leaching. The zinc sulfate solution generated in the concentrate leaching, which contains iron and rare metals, is directed to a neutralization and precipitation stage, where the solution is neutralized to a pH of 2.5-3.5 to precipitate the trivalent iron in the solution and to co-precipitate at least one rare metal with the iron. The amount of trivalent iron in the zinc sulfate solution is adjusted to 5-10% of the amount of iron in solution, corresponding to the amount required to precipitate at least one rare metal from those to be co-precipitated from the solution.

If necessary, the zinc sulfate solution is oxidized at the neutralization and precipitation stage to form a sufficient amount of trivalent iron. The solution is neutralized with at least one neutralizing agent or more than one neutralizing agent belonging to the following group: Waelz oxide, calcine, sodium hydroxide, calcium hydroxide, calcium oxide and ammonia. The precipitate of iron and at least one rare metal that forms is directed to the leaching stage to leach the rare metal and to recover by extraction. As the ferric iron content in the solution is too high for the extraction step, it is preferable to reduce some of the ferric iron back to divalent iron with some suitable reducing agent substance, which is at least one of the group. : zinc sulfide concentrate, hydrogen sulfide and sodium sulfide.

LIST OF DRAWINGS The method according to the invention is shown in the attached flow chart 1.

DETAILED DESCRIPTION OF THE INVENTION The invention relates to a method for the recovery of at least one rare metal in connection with zinc sulfide concentrate leaching. The most common rare metal in zinc sulfide concentrates is indium. Gallium behaves in leaching in much the same way as indium, so if present in the concentrate, it can also be recovered if desired at the same time. The third possible rare metal in a crude zinc material is germanium, however it partially behaves differently from indium gallium in the zinc process due to its higher degree of oxidation and requires its own type of process.

As shown by the invention in the attached flowchart 1, only the indium among rare metals is pointed out for simplicity, however the method also refers to other rare metals such as gallium. The zinc sulfide concentrate leaching step (2) (1) generally occurs at an acid concentration of 10-50 g / 1 sulfuric acid. The sulfuric acid solution is usually the acid that was used in electrolysis, which is concentrated if necessary with sulfuric acid. Additionally, oxygen-containing gas is supplied to the solution, such as air, oxygen enriched air or oxygen. When part of the concentrate is burned, the ferrite-containing leaching residue (3) that remained undissolved in the neutral calcine leaching stage can also be fed to the concentrate leaching stage if there is no separate acid leaching stage for the neutral leaching of leach residue in the zinc process. Another alternative is the ferrite residue being fed into a Waelz oven. These familiar stages, in which part of the concentrate is burned and subsequently directed to a neutral leaching step, are not presented in detail in the diagram. The sulfide concentrate leaching stage (2) generally consists of several reactors and as far as said countercurrent leaching process is concerned, it is preferable to regulate this process so that the acid concentration is higher in the first reactor and decrease in the following reactors. If the neutral leach leaching residue is also fed into the direct leaching process, it is more advantageous to use a countercurrent leaching process, which includes weak acid and strong acid leaching steps.

The results of concentrate leaching in a zinc sulfate solution (4) and a residual leach precipitate sediment (5), which mainly contain lead, silver and other precious metals in the concentrate, as well as silicon compounds, some plaster which may have precipitated and elemental sulfur. Zinc sulfate solution (4) also includes iron and rare metals, such as indium and gallium, dissolved from the concentrate. Iron is mainly in divalent form, but the leaching conditions are regulated such that 5-10% of iron is trivalent, that is, in ferric form, such that its amount corresponds to the amount required for precipitation of at least one. rare metal of solution. However, in order to minimize the amount of ferric iron in solution, because it accompanies the Indian and hinders the production of a pure Indian product. It is possible to regulate the ferric iron concentration of the solution already at the leaching stage, however if fine tuning can be done by increasing the ferric iron concentration in the neutralization step with separate oxidants such as oxygen, manganese dioxide. and potassium permanganate. The zinc sulfate-containing solution (4) leaving the zinc sulfide leaching is directed according to the invention to the neutralization and precipitation stage (6), in which the trivalent iron contained in the solution is precipitated out, whereby The desired rare metals are also co-precipitated with iron. The neutralization of the solution is performed with some appropriate neutralizing agent. If the process includes burning the concentrate, neutralization can be performed with calcine. If the process includes ferrite reduction in a Waelz furnace, the use of Waelz oxide for neutralization is especially advantageous since there are no ferites in Waelz oxide and for this reason no zinc losses are generated. If the process is not connected to the burning of the concentrate, it is preferable to neutralize with some kind of completely dissolving neutralizing agent. These are, for example, sodium hydroxide (NaOH) or ammonia (NH3) and at least part of the neutralization can be done with calcium oxide or calcium hydroxide. The pH of the solution is increased to the 2.5-3.5 range by neutralization, whereby trivalent iron is precipitated, as are indium and other desired rare metals. The pH should be set at the neutralization and precipitation stage to the correct range such that the impurities of the indium process, such as iron, are not precipitated on a large scale with it, and likewise for zinc. The purpose is that only iron in the trivalent form precipitates and the remainder of the iron is removed in a separate iron precipitation step. If the amount of trivalent iron in the solution is insufficient for precipitation of indium and other desired rare metals, the solution may be iron oxidized to ferric form. Suitable oxidizers are the family oxidants mentioned above, such as oxygen, manganese dioxide and potassium permanganate. The neutralization and precipitation stage generates a deposit (7) which contains indium and other rare metals of the concentrate which co-precipitate with iron. The obtained deposit is treated using the state of the art such that the deposit is leached in the leaching step (8) using a solution containing sulfuric acid. The solution may be a solution of sulfuric acid or spent acid from electrolysis. The obtained solution (9), which includes rare metals, ferric iron and some zinc, is directed to liquid-liquid extraction (10) to separate indium from other rare metals from impurities. If the ferric iron content in this solution is too high for economical operation of the extraction stage, the ferric iron may be reduced back, for example with zinc concentrate or with a suitable reducing agent such as hydrogen sulfide or sulfide. sodium. Extraction produces an essentially zinc-free solution from which rare metals are recovered using some method known per se to obtain an indium product. The leaching residue that precipitates in the leaching step (8) contains some lead and silver. Especially if the neutralizing agent used in the neutralization and precipitation stage (6) is Waelz oxide, it contains lead which precipitates out of solution. The neutralization and precipitation stage solution (6) is an iron-containing zinc sulfate solution (11) from which iron is precipitated into its own precipitation step (12) in some appropriate manner, typically as jarosite, goethite or hematite, and the zinc sulfate solution obtained is directed to the neutral leaching stage. The sulfate solution that comes out of indium extraction, which contains zinc, is directed via neutral leaching and purification of the solution for electrolyte zinc recovery, since the iron content is so low that it does not need to be fed to the step. iron removal. The invention is further described by way of the example below: Example 1 Indium recovery tests were divided into two stages: indium precipitation from a zinc sulfate solution produced in concentrate leaching tests and that of indium leaching tests. precipitated deposit. The purpose of the leach stage is to produce an ideal solution for later recovery of indium using liquid-liquid extraction. The execution of the method will become obvious from the following examples.

Indium Precipitation: One liter of an indium-containing zinc sulfate solution was heated to a temperature of 75 ° C in a stirred glass reactor. The mixing rate in the reactor was regulated such that the solids were kept moving throughout the test. The pH of the solution at the start of the test was 1.3 and the ferric iron concentration was 2.3 g / 1. After this, the pH was increased to 3.0 by addition of Waelz oxide (addition of 25.01 g), whereby indium and some impurities (Al, Faith, Zn) began to precipitate. The test was continued for 6 hours while maintaining the pH constant with small additions of Waelz oxide (a total addition throughout the test of 26.30 g) and slurry samples were taken after 0.5 h, 1 h, 2 h, 3 h, 4 h, and 6 h.

The samples were filtered and the indium content of the solution was analyzed. These results and the initial composition of the solution are presented in Table 1.

Table 1. Initial composition of test solution and indium concentrations at different time of precipitation test.

The results show that the Indian precipitates effectively and quickly even in the first moments of the test. At the end of the test, the total weight of the deposit was 17.72 g. 0.47 g of deposit with samples was removed during the test.

At the start of the test there were 83 mg of indium in the solution and about 5 mg at the end of the test, in other words, the percentage of precipitation was about 93%, taking into account the indium removed with the samples.

Indium-containing deposit leaching: The precipitated deposit at the previous stage was leached with a sulfuric acid solution for the purpose of producing a concentrated indium solution for the recovery of indium by liquid-liquid extraction. In the residue leaching test, 16.76 g of indium-containing deposit (0.67% In concentration) from the previous precipitation stage were mixed in a dilute sulfuric acid solution (0.5 l solution, pH 1.0 and 95 ° C) in a stirred reactor. The mixing rate in the stirred glass reactor was adjusted such that the solids remained in motion throughout the test. The test was maintained for 8 h by increasing the acid concentration to about 38 g / l at 2 h and then keeping this constant for the next 3 hours. The sulfuric acid concentration of the sludge was again increased to 50 g / l at 5 h and kept constant for the next 3 hours. Mud samples were taken after 1h, 2h, 5h and 8h.

The samples were filtered and the solution was analyzed for indium content and major impurities. These results and the solution composition at different times are shown below in Table 2. After leaching the final residue mass was 10.63 g and the In concentration 0.13%. Based on the analysis of the deposit and its mass, the indium leaching yield was about 88%. In this case, the percentage of indium yield in the total indium recovery process is around 82%.

Table 2. Composition and concentrations of indium in the test solution at different points in the leach test.

The results show that a large part of the indium dissolves during the first two hours, when the pH of the solution is 1. Increasing the acid content further increases the indium recovery even further. They also show that the solution is considerably concentrated in relation to indium when compared to the concentrations of Al, Fe and Zn with the indium concentrations in the initial solution of the precipitation test and in the final solution of the leach test.

Claims (7)

1. Method for the recovery of at least one rare metal, such as indium and / or gallium, in connection with zinc sulphate concentrate leaching, characterized by the fact that the amount of trivalent iron in a zinc sulphate solution (4) , generated in connection with the leaching of zinc sulfide concentrate (2) and containing iron and rare metals, be set to 5-10% of the amount of iron in the solution, corresponding to the amount required for precipitation of at least one rare metal. which must be co-precipitated from the solution, said solution being directed to a neutralization and precipitation stage (6), in which the solution is neutralized to a pH in the range of 2.5-3.5 in order to precipitate the iron. trivalent in solution and in order to co-precipitate at least one rare metal with iron. Method according to claim 1, characterized in that the solution is oxidized at the neutralization and precipitation stage (6) to obtain a sufficient amount of trivalent iron. Method according to claim 1 or 2, characterized in that the zinc sulfate solution (4) leaving the concentrate leaching is neutralized with one or more neutralizing agents chosen from the following: Waelz oxide, calcine, sodium hydroxide, calcium hydroxide, calcium oxide and ammonia. Method according to any one of the preceding claims 1-3, characterized in that the rare metal is indium. Method according to any one of the preceding claims 1-4, characterized in that the trivalent iron deposit and at least one rare metal formed is directed to a leaching stage (8), which is carried out by means of a sulfuric acid-containing solution to leach the rare metal. Method according to claim 5, characterized in that the leaching stage (8) of the trivalent iron deposit and at least one rare metal, part of the trivalent iron is reduced to divalent iron using a reducing agent which is at least one of the following: zinc sulfide concentrate, hydrogen sulfide and sodium sulfide. Method according to claim 4 or 5, characterized in that the solution obtained at the leaching stage of the trivalent iron precipitate and at least one rare metal is directed to extraction (10) in order to recover at least one rare metal. .